I'm no fan of Rick Warren, but the way I read that statement, he isn't speaking to just homosexual acts. It isn't a sin to love somebody. A man who loves his wife is not sinning. It also isn't necessarily a sin to have sex with someone you love: a man who sleeps with his wife is not sinning. The "it might be" statement, to my understanding, acknowledges that there are situations in which having sex with someone you love is a sin--sex outside of marriage, for example, or homosexual acts, although Warren does seem to dodge the line of questioning a bit.
Seriously, this is as bad as arguing with liberals who try to isolate down the quote or the behavior to the point where they exclude all context in order to deny what is plainly obvious about a person and their beliefs.
The context of the question did not appear to be about heterosexual relationships. Warren was trying to play both ends against the middle. His response is a masterpiece of word parsing.